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Our sex chromosomes were once just a pair of ordinary chromosomes, which they still are in birds and reptiles.
We found they are still ordinary chromosomes even in monotreme mammals (platypuses and echidnas) which last shared a common ancestor with humans 166 million years ago.
La Trobe University provides funding as a member of The Conversation AU.Most spectacularly, species in two rodent groups have lost their entire Y chromosome.Y genes have been either shunted to other chromosomes, or replaced – we don’t know by what. We can’t become a female-only species (as have some lizards, such as the New Mexico whiptail) because there are at least 30 “imprinted” genes that are active only if they come through the sperm. So does that mean humans will become extinct in 4.5 million years? The Y-less rodents have evolved a new sex determining gene, so why not humans?But the Y must contain a gene that determines maleness, because XXY people are male, and XO people with a single X but no Y are female.We know that at 12 weeks an XY human embryo develops testes, which make male hormones and cause a baby to develop as a male.
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So we are wrong if we think sex determination in human babies is typical of vertebrates.